but we ourselves will be ready armed to go before the children of
Israel, until we have brought them unto their place; and our little
ones shall dwell in the fortified cities because of the inhabitants of
the land.

What inhabitants of the land? I thought the land was completely conquered and there was no on left!

And we took all his cities at that time, and utterly destroyed every
city, the men, and the women, and the little ones; we left none
remaining;

So, why the need to fortify any cities if there were no land inhabitants?

Counter-question - if they were afraid of the inhabitants, the men are going to war leaving the women and children vulnerable to them, even if they are in a fortified city. (How strong could it actually have been?) Who was protecting them?

2 Answers
2

The Art Scroll Chumash on Mattos 32:16 cites Abarbanel who says that they intended to rebuild and refortify the cities. I have seen in a number of places that had they left those remaining behind defenseless, the "inhabitants of the land" surrounding the areas captured would have moved in and conquered the areas occupied by Reuven and Gad.

The Art Scroll Chumash commentary on Devarim 2:34 cites Ramban that, the cities of Sichon were occupied by the Amorites who were subject to the command of 20:16 that no survivors were to be left. However, the nonAmorites surrounding them were still there.

Note that only the best fighters were required to cross the Yarden. They did have regular fighters as well as the retired fighters and the youngsters not yet in the army. They would need to be able to defend the cities against attack from outside.

The youngsters just under military age seasoned by a skeleton force and trained by the "retirees" would be enough to defend the cities and prevent invasion and raiding of the flocks.

Divrei Hayamim I 5:10 and 5:19-22 mentions a war between the two and a half tribes and the "Hagarites" in the time of Shaul (and there's a Midrash - right now I don't remember the location - that says that actually it was in the time of Yehoshua). So even if the Emorites were wiped out, there were certainly other dangerous tribes and nations.